1. Field
Apparatuses and methods consistent with exemplary embodiments relate to a battery module and a method for controlling a battery module thereof, and more particularly, a battery module system that includes a plurality of battery modules which may each be controlled to switch from an active state to a bypassed state.
2. Description of Related Art
Failure of any one module in batteries consisting of multiple series-connected modules results in the failure of the series string containing the module. Thus, in batteries containing only one series string, the failure of one module will result in the failure of the entire battery. Combinations of one-shot or manual bypasses, fuses, circuit breakers, DC to DC converters, and power conditioners have been used in an attempt to mitigate the complete failure of a series string of battery modules.
For example, one-shot bypasses are used at the cell level for satellite batteries, allowing the permanent removal of any cell from the series string. Another example is the use of manual module bypasses that have been designed to allow reconfiguring of a battery to a lower voltage arrangement by partially disassembling the battery and manually bypassing the failed module. However, one shot bypasses may result in permanent reduction of the battery capability while a manual module bypass is equivalent to an undesirable in-field repair or reconfiguration.
Another issue for high power batteries is that they may have sufficient power to cause failure if a very low resistance load is connected or if a short circuit occurs. The means of protecting high power batteries from such an overload consist of fuses or circuit breakers at the string or battery level. However, these fuses or circuit breakers must be rated for the full battery voltage. Fuses and circuit breakers have undesirable behavior in many mobile systems such as hybrid electric vehicles or aviation, where the unplanned disconnection of the battery power source from the system can cause a loss of critical function such as lighting, steering, braking, or flight control. Further, high voltage fuses are physically large, making it difficult to incorporate them into modules. Such fuses are added at the string or battery level, so they are unable to interrupt the short circuit of a portion of a string, or of an individual module. It is worth also noting that batteries have output voltages that vary over a wide range of values for the state of charge and battery current. Further, such batteries may also use switches but as a standoff voltage of the solid state switching devices increases their maximum switching frequency decreases in part because of the requirement for large filters if such devices are used in DC to DC converters above a few hundred volts. Also, the nominal battery output voltage is fixed by the battery configuration and the battery capacity is fixed by the capacity of the cells in parallel.
Lithium-ion batteries in particular may have dangerously high voltage and are assembled while electrically “live” requiring operators to wear personal protective equipment. The personal protective equipment for high voltage handling limits operator dexterity and adds significant constraints to a battery design. Alternately, batteries are designed with blind-mate power connections to engage electrical connections without manual operations. However, blind mate connectors are expensive and require additional design features, such as physical interlocks to restrict operator access once any connections are engaged. Lithium Iron Phosphate batteries with parallel branches suffer from current imbalance which can lead to uneven wear-out, or in extreme cases, overload of a branch with resistance far from the norm in either direction. Further, uneven current sharing is self-perpetuating in that a string with lower resistance carries more current, thus dissipating more power, thus increasing its temperature, thus further lowering its resistance, which causes higher current, etc.
Further, another consideration is that the balance of States-Of-Charge (SOC) of modules within a series string cannot be varied except by the slow process of balancing cells. Particularly, current is shared between parallel branches of cells, modules, or strings in an array as a result of dV/dQ (open circuit voltage (OCV) versus SOC slope) and the total resistance in the branch. Electrochemical couples, such as Lithium Iron Phosphate, may be very difficult to manage due to large current imbalances between branches resulting from a large region of the OCV/SOC curve with dV/dQ≈0. Further, cell-level balancing is used to change the SOC balance between modules in a series string. However, this process is slow and typically non-regenerative, particularly; all the energy that must be moved is dissipated in a resistance.
Most high power battery systems require a dedicated circuit to precharge load capacitance. This is typically implemented with a current limiting resistor, additional contactors, and an additional fuse. Additionally, such resistive precharge current limiters will fail if load capacitance is increased, for example, during integration of customer systems. Resistive or motor loads that may be present during precharge prevent resistive current limiting and require more expensive switching precharge current limiters.
Also, DC to DC converters and power conditioners are employed in user systems to provide regulated DC power to loads. DC to DC converters are also employed to convert battery voltage to desired output voltage. However, filter components for these DC to DC converters are large, heavy, and expensive due to the low switching frequency.
Further, using some of the above noted examples, it is impossible to increase a battery's voltage by ½ of one cell's voltage, and it is difficult to vary the voltage by less than a full module's voltage. It is also impossible to increase a battery's capacity by less than one cell's capacity and undesirable to use cells of small capacity to tailor the capacity of a large battery.